I was starting getting the error Invalid AJAX response when trying to update a plugin or GRAV itself using the admin plugin. After some digging I found the following pages.
- https://discourse.getgrav.org/t/invalid-ajax-response/9239/4
- https://github.com/getgrav/grav-plugin-admin/issues/1626#issuecomment-481679453
- https://discourse.getgrav.org/t/invalid-ajax-update/11649/2
For me, and lots of others, the solution was quite simple.
- Open Settings > System > Advanced
- Change the Remote Fetch Method to fopen
- Afterwards SAVE the change and retry.